The invention relates to a device and a method for characterizing surfaces. It serves in particular to determine the crystallographic structure of crystal surfaces and to perform real-time monitoring crystal growth by molecular beam epitaxy.
The techniques most commonly used for determining the crystallographic structure of surfaces are slow electron diffraction also known as low energy electron diffraction (LEED), and diffraction by reflecting fast electrons also known as reflection high energy electron diffraction (RHEED). In particular, the RHEED technique presents the major advantage of being compatible with growing crystals by molecular beam epitaxy; as a general rule, molecular beam epitaxy apparatuses include an incorporated RHEED device. The device is constituted essentially by an electron gun arranged to produce a substantially monokinetic beam of electrons having energy of the order of 5 kiloelectron volts (keV) to 50 keV directed towards the surface under study at an angle of incidence of about 1° to 4° relative to the plane of the surface, a phosphorus screen for viewing electrons that are diffracted forwards by the surface, and a camera for acquiring images of said phosphorus screen.
The RHEED technique makes it possible to characterize the crystallographic structure of a surface completely, providing corresponding acquisitions are performed at least two distinct orientations of said surface. Nevertheless, characterization is very often limited to qualitative characterization of the state of a surface in comparison with a reference diffraction pattern. Another important application of the RHEED technique is real-time monitoring of the layer by layer growth of a crystal by molecular beam epitaxy. Once a layer has been completed, the diffraction peaks are clearly visible and present high contrast; as additional atoms become deposited on said layer, contrast worsens and begins to increase again when these atoms become sufficiently numerous to form a new layer. Oscillations are thus observed in the diffraction signal, thereby making it possible to track in real time the formation of the various layers of atoms of the crystal.
Although its advantageous properties have made the RHEED technique an industrial standard, it nevertheless presents certain drawbacks.
Firstly, even at grazing incidence, electrons present penetration power of several angstroms (Å), which means that they are sensitive not only to the first layer of atoms that strictly speaking constitute the surface, but they are also sensitive to the initial underlying layers. Furthermore, the penetration of electrons under the surface often gives rise to a diffraction pattern that is complex and that is difficult to interpret.
In addition, electron diffraction techniques (not only RHEED, but also LEED) are poorly adapted to characterizing insulating materials, since they induce a surface charge that can influence the primary beam itself and thus interfere with the diffraction pattern. Worse still, the inelastic interactions between the electrons and the surface generally damage the surface and can radically disturb the growth of insulating films. That is why those techniques do not enable the growth of insulating layers to be monitored on line, but are used rather as destructive testing techniques when devising fabrication protocols.
Given the importance of insulating layers, and in particular of oxides, in microelectronics, that is a major limitation of the technique.
In order to characterize surfaces crystallographically, it is also known to use lightweight atoms, generally of He, presenting energy of the order of a few tens or a few hundreds millielectron volts (meV) and directed perpendicularly or obliquely to the surface under study, generally at an angle of incidence lying in the range 40° to 60° relative to the plane of the surface. That technique, known as helium atom scattering (HAS) or as thermal energy atom scattering (TEAS) presents the advantage of being sensitive solely to the first layer of atoms on the sample under study, the penetrating power of low-energy atoms being negligible, and therefore not inducing and charging of insulating surfaces. Nevertheless, it is used only very rarely in industry since it presents major drawbacks.
Firstly, it is not compatible with growth by molecular beam epitaxy, which requires a large amount of space above the surface to remain empty in order to allow molecular beams to pass. Unfortunately, in order to implement the HAS/TEAS technique, it is specifically necessary to provide a source of thermal atoms not far from the normal to the surface; that technique therefore generally allows ex-situ analysis only. The LEED technique also shares this drawback, which explains why the LEED technique is less popular than the RHEED technique, even though it is superior in terms of the quality of the diffraction patterns that are obtained.
Secondly, generating beams of low-energy atoms requires the use of equipment that is heavy and bulky (supersonic jets, differential pumping stages, etc.).
Thirdly, low-energy neutral atoms are extremely difficult to detect. Detection is generally performed point by point using a mass spectrometer that is moved in two dimensions. Building up a diffraction pattern therefore requires a considerable length of time, which is not compatible with in-line monitoring.
In practice, that technique is used almost exclusively in the laboratory.
It is also known to study the structure of surfaces with the help of atoms or ions that are weakly charged and that present relatively high energy (several kiloelectron volts) at grazing incidence. Under such conditions, the projectiles behave essentially like conventional particles and they are reflected by the surface potential at a great distance from the first layer of atoms. The diffusion profile gives access indirectly to the shape of the interaction potential between the projectile and the first layer of the surface. For more details about that method of characterizing a surface, reference can be made to the article by A. Schüller et al. “Dynamic dependence of interaction potentials for keV atoms at metal surfaces”, Phys. Rev. A, 69, 050901 (R), 2004.
The drawback of that technique is that the diffusion profiles are difficult to interpret and they are always less rich in information than the profiles obtained by diffraction techniques that make use of the wave nature of the projectiles.